At first, description is made for types of antenna unit based on the conventional technology. FIG. 8 is a general view showing an example of an antenna unit based on the conventional technology, FIG. 9 is a view for explaining the principles of the antenna unit shown in FIG. 8, FIG. 10 is a general view showing another example of antenna unit on the conventional technology, FIG. 11 is a view for explaining the principles of the antenna unit shown in FIG. 10, FIG. 12 is a general view showing still another example of the antenna unit on the conventional technology and FIG. 13 is a view for explaining the principles of the antenna unit shown in FIG. 12. Applied to an antenna unit 51 shown in FIG. 8 is an Az (Azimuth)/E1 (Elevation) mount system and the antenna unit is mounted on two axes. As shown in FIG. 9, this antenna unit 51 is so constructed that it can be rotated around an azimuth angular axis 5b as well as around an elevation angular axis 51a.
Applied to an antenna unit 61 shown in FIG. 10 is a X/Y mount system and the antenna unit is mounted on two axes like in the Az/E1 mount system. This antenna unit 61 is so constructed, as shown in FIG. 11, that it can be rotated around an X axis 61a as well as around a Y axis 61b.
Applied to an antenna unit 71 shown in FIG. 12 is a HA/DEC mount system and the antenna unit is mounted on two axes like in the Az/E1 mount system or the X/Y mount system. This antenna unit 71 is so constructed, as shown in FIG. 13, that it can be rotated around a HA axis 71a as well as around a DEC axis 71b. It should be noted that, the technology on this type of antenna units 51, 61 and 71 is described in the reference Antenna Engineering Handbook (edited by Institute of Electronics and Communication Engineers) Chapter 9, Section 5.
Next, description is made for an example in which the antenna unit is applied to a satellite communication system. In recent years, there has received attention on an orbital satellite communication system in which a plurality of satellites are launched in Low Earth Orbits and data transaction is executed through communications with each of the satellites. There is a system called Teledesic in this orbital satellite communication system. In this type of system, each antenna unit located on the ground is required to seize (track) a plurality of orbital satellites one after another while the satellites are within a visible area from the ground and also it is required to continuously insure the communication routes. Namely, each of the antenna units seizes at least two satellites, monitors information for switching satellites by receiving radio waves sent from the satellites all the time, and communicates with the satellites as required through receiving and transmitting radio waves from and to the satellites.
Then, description is made for a relation between an antenna unit and a satellite. FIG. 14 is a view for explaining a positional relation between the conventional type of antenna system and orbital satellites, and FIG. 15 is a view for explaining an example of interference of radio waves in the conventional type of antenna system. In the example of FIG. 14, two antenna units 51 and 52 each having the same construction and function are installed near each other, and a large number of orbital satellites (among them orbital satellites 81, 82 and 83 are shown in the figure) orbit in Low Earth Orbit LEO. Herein the reference numeral R1 shows a radio wave between the antenna unit 51 and an orbital satellite, and the antenna unit 51 receives radio waves from a satellite and transmits radio waves to the satellite if necessary. The reference numeral R2 shows a radio wave between the antenna unit 52 and an orbital satellite, and the antenna unit 52 receives radio waves from the satellite.
The two orbital satellites 81, 82 orbiting (moving in the right direction in the figure) in Low Earth Orbit LEO are tracked by the antenna units 51, 52 respectively, and communication routes are set between the Low Earth Orbit LEO and the ground. Namely, the antenna unit 51 is tracking the orbital satellite 81, and the antenna unit 52 is tracking the orbital satellite 82. At this point of time, for example, the antenna unit 51 performs satellite communication (communications through data transaction) with the orbital satellite 81, and on the other hand the antenna unit 52 receives satellite switching information from the orbital satellite 82.
Then, when the orbital satellite 81 is out of vision from the ground, namely can not be seized by the antenna unit 51, the satellite communications are performed by switching to the communication route formed between the antenna unit 52 and the orbital satellite 82. The timing of this switching is decided according to the satellite switching information sent from the orbital satellite 82 to the antenna unit 52 before switching. This switching is executed instantly. Thus, when switching of satellites is to be performed mechanically, two antenna units 51 and 52 are required. When the satellites are switched, the antenna unit 51 starts tracking the orbital satellite 83 skipping the next orbital satellite 82. Then, for next switching of the satellites because the antenna unit 52 can not seize the orbital satellite 82 any more, the antenna unit 51 starts receiving satellite switching information, from the orbital satellite 83, for switching the communication route used for satellite communications between the antenna unit 52 and orbital satellite 82 to that between the antenna unit 51 and the orbital satellite 83.
As described above, in the next switching of satellites, the antenna unit 51 is required to switch to the orbital satellite 83 as instantaneously as possible, and at that point of time, dead angles need to be reduced as much as possible. Accordingly, when orbital satellites are tracked using the mechanical method, it is important to efficiently perform synchronization between the two antenna units 51 and 52.
In order to achieve this, it is required to provide the antenna units 51 and 52 as close as possible, but if the antenna system is so constructed that one of radio routes comes into even just a portion of the other radio route between the antenna units 51 and 52, interference of radio waves may occur between the radio route L1 in the antenna unit 51 and radio route L2 in the antenna unit 52. Therefore, a blocking portion (interfering portion) Z1 due to radio waves of the antenna unit 51 may occur in the antenna unit 52, and a blocking portion Z2 due to radio waves of the antenna unit 52 may occur in the antenna unit 51, which reduces the reliability of satellite tracking and satellite switching.
The present invention has been made for solving the problem described above, and it is an object of the present invention to obtain an antenna system in which interference does not occur even if a plurality units of antenna unit are provided at the closest possible distance between the devices and also tracking orbital satellites and switching satellites can reliably be executed.